Radial sleeve bearing and associated lubrication system

ABSTRACT

An improved sleeve bearing and associated lubrication system for the rotor shaft of a steam turbine. The bearing is lined with a metallic-polymeric composite, and is fixedly supported within a housing defining a reservoir containing a supply of liquid lubricant. The lubrication system includes a ring loosely surrounding the rotor shaft at a location spaced axially from the bearing. A lower portion of the ring is partially submerged in the lubricant. The ring is rotatably driven in response to rotation of the rotor shaft, with lubricant picked up on the ring being centrifugally discharged radially outwardly onto the surrounding housing walls. A system of communicating troughs, scuppers and passageways receives lubricant running off the housing walls for delivery under gravity to the bearing. Return passageways direct lubricant exiting from the bearing past water cooled chambers, over a weir and back to the reservoir.

BACKGROUND OF THE INVENTION

This is a continuation of copending application Ser. No. 08/602,047filed on Feb. 15, 1996 now abandoned, which is a division of Ser. No.08/399,426 filed on Mar. 7, 1995, U.S. Pat. No. 5,630,481.

1. Field of the Invention

This invention relates generally to single-stage mechanical-drive steamand gas-expansion turbines, and is concerned in particular withimprovements in the bearings employed to rotatably support the rotorshafts of such turbines, as well as to improvements in the systems usedto lubricate such bearings.

2. Description of the Prior Art

Historically, mechanical-drive turbines have been fitted either withantifriction bearings of the ball or roller type, or withhydrodynamically lubricated sliding metallic sleeve bearings.Antifriction bearings must be replaced periodically, with attendantinterruptions in turbine operation. Because of such periodic serviceinterruptions, the use of antifriction bearings is generally restrictedto relatively low-powered, smaller turbines in non-critical serviceapplications.

In contrast, if properly installed and lubricated, sleeve bearings havevirtually unlimited life. Thus, sleeve bearings have been applied to alltypes of mechanical-drive turbines, including those with higher powerratings, and in particular those operating in critical serviceapplications where process requirements cannot tolerate periodic outagesto accommodate bearing maintenance.

Turbine sleeve bearings have conventionally comprised multiple metalliclayers, with the innermost layer which bears against the rotor journalusually consisting of a babbitt material. The babbitt material is arelatively soft metal alloy of lead, tin, antimony or copper in variousproportions. Because of their relative softness, babbitt materials arecharacterized by a property commonly referred to as "imbedability", i.e.an ability to absorb reasonable quantities and sizes of foreigncontaminants such as metal particles and debris which become embedded inthe babbitt material without resulting damage to the journal. Thebabbitt materials are also considered to have "conformability", whichmeans that they tend to wear-in during initial service, therebyharmlessly accommodating minor imperfections and misalignments in thejournals and bearings.

The conventional sleeve bearings are commonly lubricatedhydrodynamically at low to moderate speeds by one or more oil ringswhich ride on the rotor journal through "windows" provided at the top ofthe bearing sleeve. The oil rings extend below the journal and sleeveand are partially submerged in a lubricant pool contained within thebearing housing. As the journal revolves, the oil ring also revolves asa result of its frictional contact with the journal surface, and in sodoing the oil ring picks up lubricant by surface tension from theunderlying pool. This oil is then deposited on the journal and isdistributed by gravity and viscous effects into the sleeve bearing whereit serves to support the rotating journal on a hydrodynamically createdoil film.

Under ideal operating conditions, the conventional oil ring-typelubrication system operates in a generally satisfactory manner toprovide adequate lubrication for the bearing. However, the higheroperating pressures, temperatures, speeds and attendant structuraldeflections of contemporary turbines have sorely taxed the capabilitiesof conventional lubrication systems, often resulting in an insufficientdelivery of lubricant to the bearing.

An inherent limitation in conventional sleeve bearing lubricationsystems stems from the fact that oil ring rotary speed is not directlyrelated to the rotational speed of the journal. More particularly, atincreasing journal speeds, the oil ring increasingly exhibits a tendencyto "slip" on the journal. It appears that such slippage results from theoil ring itself riding on an oil film on the journal, rather than beingdirectly driven by metal-to-metal traction between the ring and journal.This condition is further exacerbated by the increased force required todrag the oil ring through the underlying pool of oil. The net effect isthat at increasing journal speeds, oil ring speeds gradually level off,and as a result the oil rings become incapable of continuing to supplyenough oil to support proper lubrication and heat removal.

A further difficulty with the conventional oil ring arrangement is thatoil must be delivered to the bearing from the inside diameter of thering where it rides on the journal. However, at higher rotationalspeeds, oil is centrifuged out of the ring's inner surface and is thuscast off radially to the bearing housing walls rather than beingdelivered to the bearing where it is needed.

Insufficient bearing lubrication quickly translates into increasedbearing temperatures. The conventional babbitt materials, because oftheir softness, are relatively weak and their strength diminishesprogressively at increasing temperatures. When lubrication is marginal,or if the bearings are initially subjected to misalignment, they tend torapidly overheat and fail. The first mode of this failure is commonlyreferred to as "wiping", a term that reflects the circumferentialtearing and smearing of the distressed babbitt surface. If the source ofdistress is severe and not timely corrected, the deterioration mayprogress precipitously, with a resulting catastrophic failure of eitheror both the bearing and journal.

SUMMARY OF THE INVENTION

A general objective of the present invention is the provision of animproved turbine rotor bearing and associated lubrication system havingthe capability of operating reliably over extended periods of time atthe increased pressures, temperatures and speeds of contemporaryturbines.

A more specific objective of the present invention is to replace thebabbitt materials employed as the innermost layers of conventional allmetallic sleeve bearings with metallic-polymeric materials whichalthough known for other applications, have heretofore not beenconsidered for use as sleeve bearing materials for mechanical-drivesteam and gas-expansion turbines.

A companion objective of the present invention is the relocation of theconventional oil rings to positions axially remote from the sleevebearings, coupled with the efficient capture of oil being centrifugedfrom the rings for delivery by gravity to the bearings. By axiallydisplacing the oil rings from the bearings, it becomes possible toshorten the bearings, thereby minimizing any adverse effects resultingfrom bearing misalignments.

A further objective of the present invention is the provision ofimproved cooling and removal of entrained air from the bearinglubricant, thereby avoiding harmful lubricant hot spots, thermalstratification and short-circuit redelivery of hot oil back to thebearings.

These and other objects and advantages of the present invention will bedescribed in greater detail with reference to the accompanying drawings,wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a single-stage mechanical-drive steamturbine having bearing housings containing sleeve bearings and bearinglubrication systems in accordance with the present invention;

FIG. 2 is a sectional view on an enlarged scale taken through one of thebearing housings along line 2--2 of FIG. 1;

FIGS. 3,4, and 5 are sectional views taken respectively along lines3--3, 4--4 and 5--5 of FIG. 2;

FIG. 6 is a bottom view of the bearing cap; and

FIG. 7 is an exploded perspective view of a sleeve bearing.

DETAILED DESCRIPTION OF PREFERRED ILLUSTRATED EMBODIMENT

Referring initially to FIG. 1, a single-stage mechanical-drive steamturbine is generally indicated at 10. Turbine 10 includes a pressurecasing comprising upper and lower sections 12a, joined together by boltstypically indicated at 14. The turbine is suspended on the upper ends ofvertical support posts 16 extending upwardly from an underlying baseframe 18. The support posts 16 and base frame 18 comprise parts of asupport structure generally indicated at 20, which is described andclaimed in a copending commonly owned patent application filed on Feb.24, 1995, Ser. No. 08/393,728, U.S. Pat. No. 5,542,642 the disclosure ofwhich is herein incorporated by reference.

Turbine 10 includes a rotor shaft 22 driven by components (not shown)contained within the pressure casing. Shaft 22 is rotatably supported bybearings to be hereinafter described contained in essentially identicalbearing housings 24.

Referring additionally to FIG. 2-6, it will be seen that each bearinghousing 24 includes a base section 24a and a cap 24b joined one to theother by bolts 26 at a horizontal interface 28 containing the axis A ofrotor shaft 22. The rotor shaft is journalled for rotation in a sleevebearing 30 captured between internal integral webs 32, 34 of the basesection 24a and cap 24b. As can best be seen in FIG. 7, the bearing 30is subdivided into upper and lower halves 30a, 30b, each being providedwith laterally projecting ears 36 arranged to mechanically interengagewith the webs 32, 34 in preventing rotation of the bearing halves. Theupper bearing half 32a is provided with a top oil port 38 and side oilports 40 located above the horizontal centerline of the bearing. The oilports communicate with internal distribution grooves, one of which isshown at 41 in FIG. 7.

The bearing halves 30a, 30b are lined with a metallic-polymericmaterials of the type sold by Glacier Vandervell Ltd. of Middlesex,England under the trade marks "DU" and "Hi-eX". The DU material is ametal backed, PTFE (Polytetrafluoroethylene) and lead lined composite.Hi-eX is a steel backed composite bearing material lined with PEEK(polyether ether ketone) along with various fillers including PTFE. Suchmaterials offer all of the advantages of conventional babbitt materials,while obviating or at least substantially minimizing any of theirassociated drawbacks and deficiencies. More particularly, themetallic-polymeric materials have equal or better imbedibility andconformability characteristics as well as self-lubricating propertieswhich allow the bearings to run dry for indefinite periods. Thisprovides undiminished performance, load capacity and reliability duringhigh-rate turbine startups and during operation following lengthyperiods of shutdown. These metallic-polymeric materials have extremelyhigh continuous operating temperature limits ranging up to 536° F. andare therefore unaffected by hot shutdown extremes, well past theignition point of the most common turbine lubricating oils and past themelting point of all babbitt bearing alloys. This also means that underhigh bearing misalignment conditions caused by high turbine operatingpressure and high thermal deflections, the metallic-polymeric materialswill survive transient abnormal bearing edge-loading conditions untilthe bearing re-beds itself. Such materials will therefore surviveconditions under which a babbitt bearing would fail through wiping,seizure and localized meltdown, e.g. a catastrophic failure.

Such materials are unaffected by and thrive on straight water orwater-oil emulsions, and are thus highly resistant to deterioration fromcontamination of lubricating oil by influx from broken or neglected andworn turbine steam and atmospheric seals.

Turning now to a description of the bearing lubrication system, the basesection 24a of the bearing housing is internally configured to define anoil reservoir containing a supply of oil 42 at a level 44. As can bestbe seen in FIG. 2, the reservoir includes paths Pa, Pb leadingdownwardly from opposite ends of the bearing 30 to a lower connectingpath Pc which in turn leads to a downward vertical path P_(d) connectingwith a lower horizontal path P_(e). The horizontal path P_(e) leads to avertical path P_(f) which extends upwardly to a weir 46. The pathsP_(d), P_(e) and P_(f) lead past chambers 48 and/or 50 through whichcooling water is constantly circulated.

A drive collar 52 is tightly fitted on the rotor shaft 22 for rotationtherewith. An oil ring 54 loosely surrounds the rotor shaft 22 and drivecollar 52 at a location spaced axially from the bearing 30. The oil ringdepends from the collar 52 and is partially submerged in the oil 42contained in the underlying reservoir.

The bearing cap 24b has an upper recess 56 spanned by a trough 58 spacedabove the oil ring 54 and leading to a horizontal passageway 60 in theinternal web 34. Passageway 60 in turn communicates with a verticalpassageway 62 leading to the top oil port 38 in the upper bearing half30a.

As is best shown in FIG. 3, the upper cap recess 56 has arcuate roofsegments 64 with adjacent ends which meet as at 66 at a location spacedabove the trough 58, and with opposite ends which lead to side scuppers68. As can best be seen in FIG. 6, the side scuppers 68 have parallelsegments 68a, diagonally inwardly converging segments 68b, and laterallyinwardly extending segments 68c. As can best be seen in FIG. 4, thescupper segments 68c communicate with the oil side ports 40 in the upperbearing half 30a.

During turbine operation, the oil ring 54 is revolved by the drivecollar 52 which is tightly fitted to the turbine rotor shaft 22. The oilring is partially submerged in the turbine lubricating oil 42 which ismaintained at oil level 44. Oil is picked up on the ring 54 by capillaryattraction (surface-tension) and then shed from the ring undercentrifugal force, spraying in all radial directions and resulting inoil impinging on the arcuate roof segments 64 of the bearing cap 24b.Oil runs down the roof segments 64 under gravity into the central trough58 and into the side scuppers 68. The trough 58 and its communicatingpassageways 60,62 are arranged to deliver a gravity flow of oil to thetop oil port 38 of the bearing 30. Similarly, the side scuppers 68 aredesigned to deliver a gravity flow of oil to the side ports 40 of thebearing 30.

Lubricating oil enters the upper bearing half 30a through ports 38 and40 where it serves to hydrodynamically support the rotating shaft 22before exiting into the reservoir paths P_(a), P_(b). From here, the oilcontinues to flow under gravity along paths P_(c), P_(d) and P_(e)before rising along path P_(f) and passing over weir 46 to again arrivein the vicinity of the submerged portion of the oil ring 54. As the oilflows along paths P_(d), P_(e) and P_(f), it is cooled by the coolantbeing circulated through chambers 48, 50. As the oil makes the turn fromvertical to horizontal flow over the weir 46, any buoyant entrained airand foam will continue to ascend and will ultimately effervesce at thesurface 44.

In light of the foregoing, those skilled in the art will appreciate thatthe present invention offers a number of significant advantages. Forexample, the lower bearing half 30b, which is the most heavily loadedportion of the bearing, is uninterrupted by oil ports and grooves, andthus offers full bearing area and capacity. This, when combined with theremote positioning of the oil ring 54 outside of the bearing confines,makes it possible to design the bearing with unique short proportionsfor enhanced tolerance to bearing and housing misalignment. Indeed, withthis bearing design, the ratio of bearing length L to bearing diameter Dcan be expressed as:

    L/D≦0.75

The axially remote positioning of the oil ring and its associated oilcollection system, including the trough 58, arcuate roof segments 64 andside scuppers 68, takes advantage of the oil being centrifugally castoff the oil ring, and thereby provides a lubricating oil delivery systemwhich increases delivery of oil to the bearing in direct proportion toincreasing rotational speed of the turbine rotor shaft 22.

All of hot oil exiting from the bearing 30 must return in aunidirectional path past the water cooled surfaces of chambers 48, 50,thereby eliminating any possibility of stratification, stagnation andthe creation hot spots. All oil returning to the vicinity of the oilring 54 must pass over the weir 46, thus providing for optimaldeaeration. Thus, short circuiting of hot air laden oil from bearing 30back to the vicinity of the oil ring 54 is eliminated, therebymaximizing cooling efficiency.

It will be understood that various changes and modifications may be madeto the above described embodiment without departing from the spirit andscope of the invention as defined by the appended claims. For example,multiple oil rings may be employed in place of the single disclosed oilring 54. The disclosed lubrication system may be advantageously employedwith bearings lined With other materials, including the conventionalbabbitt materials.

I claim:
 1. In an oil lubricated bearing assembly for the rotor shaft of a steam turbine, a sleeve bearing for rotatably supporting said shaft, said sleeve bearing being fixedly supported within a bearing housing defining a reservoir containing a supply of liquid lubricant, said sleeve bearing being spaced axially from a ring loosely surrounding and rotatable relative to said shaft, said ring being partially submerged in said lubricant and being rotatably driven in response to rotation of said shaft, with lubricant picked up on said ring through capillary action being centrifugally discharged from said ring for delivery to said bearing, the improvement comprising: said bearing having a L/D ratio of ≦0.75 where:L=axial length of said bearing D=internal diameter of said bearing.
 2. The sleeve of claim 1 wherein the bearing is subdivided into upper and lower segments, and wherein said upper segment is provided with lubricant inlet ports, and said lower segment is continuous and uninterrupted by such ports.
 3. The sleeve bearing according to any one of claims 1 or 2 wherein said bearing is lined with a metallic-polymeric composite.
 4. The sleeve bearing of claim 3 wherein said metallic-polymeric composite includes polytetrafluoroethylene. 